Some refrigerant compressors for vehicle air conditioner have a power transmission mechanism 100 as illustrated in FIG. 4 (for example, refer to Japanese Laid-Open Patent Publication No. 60-175830), which intermittently transmits power of a vehicle engine to the rotary shaft of the compressor. That is, the refrigerant compressor includes a fixed frame 101 and an input hub 102 provided outside of the fixed frame 101. The input hub 102 receives power from a vehicle engine (not shown). The input hub 102 serves as a first rotating body and is rotatably supported by the fixed frame 101 with a bearing 103. A excitation coil 104 is accommodated in the input hub 102. The input hub 102 has an inner circumferential surface 102a at its distal end.
An output hub 105 serving as a second rotating body is provided at an end of an output shaft 106 of the refrigerant compressor. The output hub 105 includes a first hub member 107 fixed to the output shaft 106, a second hub member 108 fixed to the first hub member 107, and a spring retainer drum 109 fixed to the second hub member 108. The second hub member 108 has an inner circumferential surface 108a on a surface that is opposite to its outer circumference. An armature 110 is provided on the first hub member 107. The armature 110 faces the input hub 102 with a space in between. A coil spring 111 is wound about the outer circumferential surface of the spring retainer drum 109. A first end of the coil spring 111 is connected to the output hub 105, specifically, to the spring retainer drum 109. A second end of the coil spring 111 is connected to the armature 110.
When power is transmitted from the vehicle engine to the rotary shaft 106 of the refrigerant compressor using the power transmission mechanism 100, a current is supplied to the excitation coil 104, so that the excitation coil 104 is excited. This causes the armature 110 to adhere to and be pressed against the input hub 102. Accordingly, the armature 110 and the input hub 102 rotate integrally. Also, the coil spring 111 rotates at the second end, which is connected to the armature 110. In this state, since the coil spring 111 receives a rotational force in the unwinding direction, the coil spring 111 is radially outwardly expanded with respect to its rotation axis, or the axis of the rotary shaft of the compressor. That is, coil diameter of the coil spring 111 is increased. The expanded coil spring 111 presses itself against the inner circumferential surface 102a of the input hub 102 and the inner circumferential surface 108a of the output hub 105 (that is, the second hub member 108). Thus, the power from the vehicle engine is transmitted from the input hub 102 through the coil spring 111 to the second hub member 108, and then transmitted through the output hub 105 to the output shaft 106.
In the power transmission mechanism 100, the coil spring 111 with an increased diameter is pressed against the inner circumferential surface 102a of the input hub 102 and the inner circumferential surface 108a of the second hub member 108, so that power is transmitted. To transmit power by means of such pressing of the coil spring 111, a desired length of the coil spring 111 needs to be pressed against the two inner circumferential surfaces 102a, 108a. That is, the inner circumferential surfaces 102a, 108a need to have a certain length in the axial direction, which increased the size of the power transmission mechanism 100. Pressing the coil spring 111 with an increased diameter against the inner circumferential surfaces 102a, 108a wears the coil spring 111 and both circumferential surfaces 102a, 108a. This reduces the reliability of the power transmission mechanism 100.